Brown dwarfs (BD) are called 'failed stars', but are important on the study of atmospheres of exoplanets, and the initial mass function. However, the BDs population is difficult to characterise. First because BDs cool/fade and change temperature/luminosity over time, and caused the mass-age degeneracy. Secondly, the observational differences between low mass stars and brown dwarfs were blurred by transitional BDs (T-BD). The existence of T-BDs was ignored by most observers. The STZ formed a mass range of the substellar transitional zone (STZ), which separates very low-mass stars, T-BDs, and degenerate BDs (D-BD). T-BDs have unsteady hydrogen fusion in their cores to replenish the dissipation of their initial thermal energy, thus have extremely slow cooling rate. As the majority of BDs, D-BDs have no energy supply from hydrogen fusion, thus cool continuously. The STZ has a narrow mass range but could be stretched into a wide range of temperature/luminosity over time. Therefore, the STZ could be resolved in the metal poor BD population, which are all very old (~10 Gyr) but extremely rare. I will present the different evolutions and observational properties (correlations between spectral types, colors, absolute magnitudes) of very low-mass stars, T-BDs and D-BDs. I will also show how metal-poor brown dwarfs lead us to new interpretation of evolutionary models, which in return help us to reach a better understanding of brown dwarf population. This poster is based on works published in a series titled 'Primeval very low-mass stars and brown dwarfs' (https://ui.adsabs.harvard.edu/public-libraries/gVGomDWcQGyKPWw2CGg3dg).